The equilibrium compositions of high-silicon metallic melts coexisting with CaO–Al2O3–SiO2 slag are determined. The dependence of the activity a of silicon in ferrosilicon melts is formulated as activity isolines on the Fe–Si phase diagram. The dependence of log $${P_{{O_2}}}$$ on x Si and T is determined at 1823, 1873, and 1973 K. The influence of $${a_{Si{O_2}}}$$ which depends on x Si and T, on the equilibrium composition of the gas phase $${P_{{O_2}}}$$ is studied. Data on the activity of silicon dioxide in CaO–Al2O3–SiO2 slags is analyzed and corrected. At fixed temperature, each alloy of specified composition corresponds to only one value of $${a_{Si{O_2}}}$$ represented on the concentration diagram by an activity isoline. In the presence of carbon, all the high-silicon alloys may be obtained at T ≥ 1973 K, with $${a_{Si{O_2}}}$$ > 0.8. At 1873 K, when $${a_{Si{O_2}}}$$ > 0.9, only alloys with <25% Si may be obtained. With decrease in $${a_{Si{O_2}}}$$ to 0.4, the limiting Si content is 20%. At lower temperatures (≤1823 K), only low-silicon alloys (<15% Si) may be obtained under acidic slag ( $${a_{Si{O_2}}}$$ > 0.5).